My friend converted his Marshal guitar amp (he had a tube amp repairman do it) to a stereo audio amp. The Marshal amp has 12AX7 driver tubes and he would like to try different pre-amp tubes.
The high frequencies (above ~14Khz) are weak meaning at least 6db too low and the mids are slightly grainy. But the bass and mid bass are great.
1) What kind of tubes will make it sound less like a guitar amp and more like a HiFi audio amp?
2) are there any tubes that are a simple direct replacement so that he doesn't have to take it back to the tube tech for a mod?
Eddie on behalf of Joe M.
The high frequencies (above ~14Khz) are weak meaning at least 6db too low and the mids are slightly grainy. But the bass and mid bass are great.
1) What kind of tubes will make it sound less like a guitar amp and more like a HiFi audio amp?
2) are there any tubes that are a simple direct replacement so that he doesn't have to take it back to the tube tech for a mod?
Eddie on behalf of Joe M.
That was a very, very bad idea.
Just by changing some tubes, you won't get the missing treble back. A competent technician should review the schematic and fix it. The output transformers may need to be changed, too...
As Vincent77 says it wont be the tubes, but the whole circuit topology. Especially the output transformers which probably won't pass the higher frequencies. Guitar speakers normally cut off at about 5kHz, so the amp will be made to suit.
Output transformers are the key , marshall amps always sounded a lot midrange focused and this is by their non flat response...and the bass region can't be that good.
Guitar amps are good from 60Hz to 6KHz , hifi is at least 20Hz to 20KHz 🙂
Guitar amps are good from 60Hz to 6KHz , hifi is at least 20Hz to 20KHz 🙂
Guitar amps often use very high values for grid stopper resistors, as compensation for the appalling wiring layout and resultant instability. These create a low HF rolloff, which can then be adjusted a little by 'tube rolling'.
To correct the problem you need to rebuild the amp to a different circuit, different layout and different components - as others have said.
To correct the problem you need to rebuild the amp to a different circuit, different layout and different components - as others have said.
But... why? Marshalls are great amps, especially the tube ones, and especially those that would be simple enough to convert. They're not at all suited to reproducing high-fidelity audio. These are amps which sound best when cranked up and distorting in a glorious fashion, and what you've done makes the guitarist in me want to cry.
I suspect something like this: Rack Power Amps | Marshall Amps
EL34 50/50 or EL34 100/100 rack mount stereo amps. I had a friend who had a 100/100...
Not sure why you would not just go out and get a pair of VTL 100s or similar..
EL34 50/50 or EL34 100/100 rack mount stereo amps. I had a friend who had a 100/100...
Not sure why you would not just go out and get a pair of VTL 100s or similar..
+1^^
the output transformer of marshall amplifiers has a cutoff at approx 15Khz.
the preamp stages are designed to be overdriven and biased hot.
there is an HPF integrated in preamp signal chain before it reaches the tone stack and PI.
i guess you won't rebuild it...
my best suggestion is to put a 12AU7 in all the preamp stages and in the PI.
they have a lower gain and might give you a smoother not distorted frequency response.
i just hope it wasn't a plexi...
PLEASE tell me it wasn't a plexi!
The puzzling part is that someone not only thought this was a good idea, but now wants to "fix" the predictable problems. Weird.
the Marshal amp is the 9200, with four transformers for a monoblock setup but all in one chassis.
To begin with, replace those guitar output transformers with proper wideband low distortion Hi Fi types.
Only *after* that you can start worrying about circuit, tubes, etc.
Only *after* that you can start worrying about circuit, tubes, etc.
Man, the 9200 is a beast and as a guitar player i would keep it as is or if you don't want to use it anymore just sell it and buy some quality transformers.
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